1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst. More specifically, this invention relates to the routes of catalyst transfer and feed and catalyst contacting.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
Despite the long existence of the FCC process, techniques are continually sought for improving product recovery both in terms of product quantity and composition, i.e. yield and selectivity. Two facets of the FCC process that have received attention are recovery of adsorbed products from the spent FCC catalyst and initial contacting of the FCC feed with the regenerated catalyst. Improvement in the recovery of hydrocarbons from spent catalyst directly improves yields while better initial feed and catalyst contacting tends to benefit yield and selectivity.
A variety of devices and piping arrangements have been employed to initially contact catalyst with feed. U.S. Pat. No. 5,017,343 is representative of devices that attempt to improve feed and catalyst contacting by maximizing feed dispersion. Another approach to improved feed and catalyst contacting is to increase the penetration of the feed into a flowing stream of catalyst. U.S. Pat. No. 4,960,503 exemplifies this approach where a plurality of nozzles rings an FCC riser to shoot feed into a moving catalyst stream from a multiplicity of discharge points. While these methods do improve feed distribution of the feed into the hot regenerated catalyst stream, there is still a transitory period of poor distribution when the relatively small quantities of the hydrocarbon feed disproportionately contact large quantities of hot catalyst. This poor thermal distribution results in non-selective cracking and the production of low value products such as dry gas.
A different approach to feed and catalyst contacting reduces local temperature maldistribution when mixing hot catalyst with the feed. U.S. Pat. No. 4,960,503 teaches indirect heating of the feed with the hot catalyst before contacting the feed with the regenerated catalyst in a reaction zone. By raising the temperature of the feed, less feed heating is required as the catalyst and feed are combined. Heating of the feed by indirect heat exchange with the catalyst can cause coking in the heat exchange equipment.
In addition to improving initial feed and catalyst contacting, more complete stripping of hydrocarbons from the spent catalyst offers further advantages. More complete stripping removes hydrocarbons from the catalyst that are lost by combustion when the spent catalyst enters the regeneration zone. Common methods to more completely strip catalyst raise the temperature of the spent catalyst in the stripping zone as a means of desorbing hydrocarbons from spent catalyst prior to regeneration. One system for heating spent catalyst in the stripping zone employs indirect heat transfer. A more common method of heating spent catalyst in the stripping zone mixes higher temperature regenerated catalyst with the spent catalyst in the stripping zone. U.S. Pat. Nos. 3,821,103 and 2,451,619 describe systems for direct heating of spent catalyst with hot regenerated catalyst.
Aside from increasing hydrocarbon recovery, reducing the carryover of hydrocarbons into the regeneration zone improves the overall heat balance of the FCC unit. Hydrocarbon that enters the regeneration zone releases additional high temperature heat as it bums in the oxygen atmosphere. Any additional heat release in the regenerator interferes with the regenerator operation by raising temperatures in the regeneration zone or requiting cooling methods to maintain a suitable temperature.
The processing of increasingly heavier feeds and the tendency of such feeds to elevate coke production makes the control of regenerator temperatures difficult. Optimization of feedstock conversion is ordinarily thought to require essentially complete removal of coke from the catalyst. This essentially-complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 weight percent coke. In order to obtain complete regeneration, oxygen in excess of the stoichiometric amount necessary for the combustion of coke to carbon oxides is charged to the regenerator. Excess oxygen in the regeneration zone will also react with carbon monoxide produced by the combustion of coke, thereby yielding a further evolution of heat.
Apart from the objective of minimizing dilute phase CO combustion, the increase in coke on spent catalyst results in a larger amount of coke being burned in the regenerator per pound of catalyst circulated. Heat is removed from the regenerator in conventional FCC units in the flue gas, and principally in the hot regenerated catalyst stream. An increase in the level of coke on spent catalyst will increase the temperature difference between the reactor and the regenerator, and the regenerated catalyst temperature overall. A reduction in the amount of catalyst circulated is, therefore, necessary in order to maintain the same reactor temperature. However, as discussed above the lower catalyst circulation rate required by the higher temperature difference between the reactor and the regenerator will lower hydrocarbon conversion, making it necessary to operate with a higher reactor temperature in order to maintain conversion at the desired level. This will cause a change in yield structure which may or may not be desirable, depending on what products are required from the process. Also, there are limitations to the temperatures that can be tolerated by FCC catalyst without having a substantial detrimental effect on catalyst activity. Generally, with commonly available modem FCC catalyst, temperatures of regenerated catalyst are usually maintained below 760.degree. C. (1400.degree. F.), since loss of activity would be very severe at about 760.degree.-790.degree. C. (1400.degree.-1450.degree. F.). If a relatively common reduced crude such as that derived from Light Arabian crude oil was charged to a conventional FCC unit, and operated at a temperature required for high conversion to lighter products, i.e., similar to that for a gas oil charge, the regenerator temperature would operate in the range of 870.degree.-980.degree. C. (1600.degree.-1800.degree. F.).
Restricting catalyst circulation to the reactor side of the FCC process affects more than yield structure of the products. The circulation rate of catalyst to the reactor influences the catalyst circulation rate through the regenerator. A decrease in the circulation of catalyst to the reactor can also lower the overall catalyst circulation rate through the regenerator. The use of additional conduits such as a recirculation line that transfers catalyst from the outlet of the regeneration zone to the inlet of the regeneration zone can reduce the interdependency of catalyst circulation through the reactor and regeneration zone. However, the use of a recirculation conduit complicates regulation of the catalyst circulation through the process and necessitates the maintenance of additional catalyst inventory on the reactor and regenerator side of the process to provide a buffer for variations in catalyst circulation. Thus, the reactor and regenerator operate with two interdependent catalyst circulation loops.
It is an object of this invention to improve the initial mixing of catalyst with the hydrocarbon feed in an FCC process.
It is a further object of this invention to fully integrate the mixing of regenerant and spent catalyst into a hot stripping operation for an FCC process.
A further object of this invention is to decouple the catalyst circulation on the regeneration side of the process from catalyst circulation on the reactor side of the process.
Another object of this invention is to reduce the amount of catalyst inventory maintained in an FCC process.